The mammalian cerebral cortex is composed of a complex, highly organized array of neuronal and glial cell types which arise from a layer of neuroepithelial stem cells. The long term goal of these studies is to understand how the diverse cell types of the cerebral cortex arise from this pool of progenitor cells, and in particular what influence a cell's lineage has on its developmental choices. The role of cell lineage in the determination of mature phenotype will be studied by labeling cells with a recombinant retrovirus (which is replication defective and contains genes that manufacture readily detectable products) and examining the progeny of these cells. The proposed research will combine retroviral lineage tracing with electron microscopy to study the phenotype and position of clonally related cells. (1) I will determine the lineage relationships of the different types of neurons and glia in the cerebral cortex. The position and mature phenotype of clonally related cells will be examined following the injection of a retroviral lineage tracer into the cerebral ventricles of rat embryos at a variety of ages. To verify that identified cells belong to a clone,the polymerase chain reaction will be used to establish that cells in a presumed clone share a common retroviral integration site. (2) Additional studies will analyze the acquisition of cell phenotype during development, with particular attention to the influence of lineage on this process. Changes in the composition and size of clones during development will be measured and used to infer whether or not lineage exerts an influence on the fate of cells in the cerebral cortex. (3) Other experiments will assess the influence of environment on the acquisition of cell phenotype. To study the regulation of cell differentiation, purified, retroviral-labeled glial cells from the developing optic nerve (identified as asemytes, oligodendrocytes, or glial progenitor cells) will be injected into the developing cerebral hemispheres, and the phenotype of these cells and their progeny will examined at a variety of subsequent times. This analysis of the role of cell lineage in the cerebral cortex will enhance our understanding of how cells make developmental choices during the formation of the mammalian central nervous system, and will serve as a foundation for the elucidation of genetic controls on the generation and regulation of cell diversity during cerebral corticogenesis.